Steam zucchini, cauliflower, peas, and carrots until crisp-tender.
Drain and season with salt.
Cool.
Melt margarine in a skillet; add flour and stir for 1/2 a minute.
Gradually add hot milk, stirring constantly until sauce thickens.
Season sauce with onion.
Cool.
Beat egg yolks into sauce and add salt.
Fold in beaten egg whites, the cooled vegetables and sauteed mushrooms.
Pour mixture into a greased souffle dish.
Place dish in a pan of hot water and bake in a preheated 325 degree oven for 1 1/2 hours, or until souffle is well puffed and lightly browned.